The Leviathan story should have been a lot more interesting than it was. Jonathan Frid, for instance, was turning in his best performances in ages, the stuff with the kid is deeply creepy, and some of the long-term links with 1897 were fascinating, particularly Julia tracking down Tate.
Problem is, we don't really get that story. It starts, then the writers seem to lose confidence, and begin wildly changing track, with the result being a very random bunch of half-stories that don't really deliver what the early episodes promise.
I think also the writers didn't do much to ease the transition after spending *way* too long in 1897. The show had hit its peak viewing figures during that time, so there were segments of the audience who had never experienced DS in the present day. They make very few concessions for the casual audience - after nine months away, they really needed to restart the present day story afresh and reintroduce the characters properly.
Pity - it was a nice opportunity to get back-to-basics and try a fresh angle. It could have worked great with some subtle tweaks and a little more long-term investment.